More than half of all Americans and about three out of four children have now survived a case of COVID-19, new data from the Centers for Disease Control and Prevention suggests after the record rise in cases driven by the Omicron variant over the winter.
The new findings were published Tuesday in the CDC’s Morbidity and Mortality Weekly Report. The study’s authors estimated “seroprevalence” caused by a previous infection – the presence of antibodies to the virus – based on samples collected from thousands of routine blood tests around the country.
Seroprevalence is significantly higher when one also includes individuals who have antibodies from vaccination. By December, the CDC had estimated that “combined seroprevalence” between the two groups was already 95%.
“We all know that there are some infections that are not reported, either because they are asymptomatic or mild, the person was not tested or it was not reported. So seroprevalence was an important piece in the puzzle and it helps us to really to understand more kind of picture of public health, “told CDC’s Dr. Kristie Clarke to reporters Tuesday.
CDC Director Dr. Rochelle Walensky acknowledged Tuesday that the rising use of quick home tests also resulted in an increasing number of unreported cases, but said the agency was still convinced it had a “reliable picture” of COVID’s trajectory in the country.
“Of course, we continue to follow trends in positive PCR tests, and we’ve also looked at data on a select percentage of rapid tests. The trends are pretty much the same,” Walensky said.
While the agency officially counted 54 million reported cases by the end of last year, the agency’s seroprevalence estimates indicated that the true figure was more than double that number.
Now, the study estimates that the Omicron wave nearly doubled the proportion of Americans with antibodies from a previous infection: seroprevalence among all Americans rose from 33.5% in December to 57.7% in February.
The age group with the largest proportion of previous infections are children. For children under 12 years of age, seroprevalence increased from 44.2% to 75.2%. In adolescents 12 to 17 years of age, seroprevalence increased from 45.6% to 74.2%.
Increases in seroprevalence have also been recorded abroad.
Up to and including February, Canadian blood services estimated that about 23.7% of donors had antibodies from an infection – almost quadrupling the 6.4% recorded in December. In Scotland, authorities last month estimated that seroprevalence from a previous infection was at least 44.5% – up from at least 29.3% in mid-January.
Out of two areas in South Africa, a CDC magazine has recently published results suggesting that seroprevalence in the wake of the Omicron wave rose to 60% in a rural area and 70% in an urban community.
Clarke said CDC researchers were also working on a study that will soon be released as a pre-print estimating the new true number of COVID-19 infections.
“In the period that covered December, January, February, such a kind of Omicron period, we found that the ratio of infection to cases was the highest it has been. Over three estimated infections per reported case,” Clarke said .
Turning up antibodies from past infections does not mean that all of these Americans are protected from future cases. During the Omicron wave, the CDC reported cases of people surviving a Delta variant infection and appearing to be re-infected by Omicron less than three months later.
Officials on Tuesday repeatedly warned that signs of a previous infection did not affect most of their instructions to take precautions around COVID-19.
However, federal health officials have repeatedly cited the widespread combined immunity to previous infection and vaccination by making predictions that the renewed increase in cases driven by BA.2 subvariant of Omicron may not result in a repeat of the overwhelming hospitals and morgues of the winter.
While the BA.1 subvariant of Omicron had dominated new infections through February, BA.2 has steadily increased in prevalence and now accounts for virtually all circulating viruses. More than a quarter nationwide is now attached to a specific tribe called BA.2.12.1, which has driven a wave of cases and hospitalizations in the Northeast.
“Further evaluation is currently underway to understand the impact of BA.2.12.1 on the effectiveness of the vaccine. But it is important that we continue to believe that those who have been vaccinated, and especially those who have been boosted, continues to have strong protection against serious illness, even from BA.2.12.1, “Walensky said.